Apple also didn't say how long these books will remain free, so you may want to check out the sale in the Books app as soon as you can.
Is Apple providing Apple Books free in other countries as well?
If you tap the notification, Apple will open the Free Books section in the Apple Books app. Apple also said, "Discover free books, reading for kids, mysterious topics and audiobooks for the whole family." says. At the top you can swipe to see featured collections. You can browse through several books from various genres, explore a wide selection of audiobooks, or select a new book from Apple's "First in a Series, Free" tab. If you can not find it, wait for Apple Inc to provide the service in your country.